I. Field of the Invention
This invention relates to derrick rigs used for attaching drill tubing to or removing drill tubing from a drill string at a well head.
II. Description of the Prior Art
The conventional way of lengthening or shortening a drill string made up of interconnected pipe sections at a well head is by means of a derrick, i.e. a framework tower constructed over the well head and including a hoist for raising or lowering the drill string. Once the drill string has been raised or lowered by a distance corresponding to the length of one or more pipe sections, the drill string is clamped at the well head and either projecting pipe sections are removed (if the drill string is being shortened) or new pipe sections are added (if the drill string is being lengthened), and the process is repeated.
This procedure requires an operator, usually referred to as a derrickman, to transfer the pipe sections between the drill string and a storage station for the pipe sections. This is inefficient and can be dangerous for the derrickman.
Attempts have been made in the past to automate this procedure to avoid the need for a derrickman. However, these attempts have generally required an operation in which pipe sections have to be laid out on the ground, where the pipe couplings may be contaminated with mud or the like, and in which single pipe sections are handled individually.